Turbo 350 Transmission Mods

Tech Team
Join Date: Jan 2005
Location: new york city
Posts: 224
Re: Mods “required” to make a th350 last with 500hp

Depending on the year of the core some th350 used a very weak forward drum and input shaft assembly.The problem is the shaft is too hard and snaps or the drum strips out.Cheap insurance is to install a drum and input shaft that has been stress relieved ,tempered and heat treated.The intermediate sprag outer race failure is very common and can be aggravated by a valve body without proper 2nd gear clutch pack accumuation designed into it.Cheap insurance is to install a 40 element direct drum and sprag assembly.The 40 element is stronger than all others including b&m,tci,bte etc. 34 element setup. And in the event of a 40 element sprag failure(ive yet to see one) the sprag is replaced with a 35.00 item and not the $125.00 job they will try to sell you.I dont believe in the softened outer race commercially available with anything over 350 horsepower and a valve body of unknown origin or improper calibration.Clutch selection should be 5 forward frictions with stock lining,3 intermediate frictions with stock lining and 5 directfrictions with red linind. It may be necessary to machine the direct clutch piston to install 5 friction plates.This is done on a machinist or brake lathe.Dual feed the direct clutch by removing the middle ring from the stator support and installing a plug in the pump face of the case in the reverse circuit.Use a stock seperator plate and valve body with all 4 ballchecks installed,drill the 2 clutch feeds in the plate to .110″,plug the third accumulator hole in the valve body casting (dont remove the spring or you risk breaking off the band lug when manual shifting),install a hi rate pressure regulator spring and a screen filter.Locate the 350 lockup front and rear planets as they have needle bearing assemblies instead of thrust washers to free up horsepower.Machine the low reverse roller clutch inner race .140″shorter and install a a500 converter bearing in place of the plastic washer.Drill an .060″ air bleed in the ineer area of the reverse piston to reduce bindup on manual 1/2 upshifts.Install an oversize direct drum bushing in the drum to better stabilize it on the support and reduce wobbling.If you want later wide open throttle upshifts without losing part throttle passing gear install a special light 700r4 governer from an 86 to 88 9ci coded Caprice police car . The late 4l60e low reverse steels with turbulator holes can be installed in the low reverse section to reduce clutch drag at high rpm and the resulting power absorbtion they cause as well as installing the larger 4l60 low reverse sprag assembly .The early 70s sunshells are much thicker and tougher than the late wimpy ones so try to find one if you can. They are usually .210″thick.That should be all you need.If you are intersed in any of the specialty drum and shafts or anything else for the rebuild check out our listings on the site.

www.ckperformance.com christoforos kokkonis gm transmission